A factor leading to development of vascular disease, a leading cause of death in industrialized nations, is elevated serum cholesterol. It is estimated that 19% of Americans between the ages of 20 and 74 years of age have high serum cholesterol. The most prevalent form of vascular disease is arteriosclerosis, a condition associated with the thickening and hardening of the arterial wall. Arteriosclerosis of the large vessels is referred to as atherosclerosis. Atherosclerosis is the predominant underlying factor in vascular disorders such as coronary artery disease, aortic aneurysm, arterial disease of the lower extremities and cerebrovascular disease.
Cholesteryl esters are a major component of atherosclerotic lesions and the major storage form of cholesterol in arterial wall cells. Formation of cholesteryl esters is also a step in the intestinal absorption of dietary cholesterol. Thus, inhibition of cholesteryl ester formation and reduction of serum cholesterol can inhibit the progression of atherosclerotic lesion formation, decrease the accumulation of cholesteryl esters in the arterial wall, and block the intestinal absorption of dietary cholesterol.
The regulation of whole-body cholesterol homeostasis in mammals and animals involves the regulation of intestinal cholesterol absorption, cellular cholesterol trafficking, dietary cholesterol and modulation of cholesterol biosynthesis, bile acid biosynthesis, steroid biosynthesis and the catabolism of the cholesterol-containing plasma lipoproteins. Regulation of intestinal cholesterol absorption has proven to be an effective means by which to regulate serum cholesterol levels. For example, a cholesterol absorption inhibitor, ezetimibe
has been shown to be effective in this regard. A pharmaceutical composition containing ezetimibe is commercially available from Merck/Schering-Plough Pharmaceuticals, Inc. under the trade name Zetia®. Identification of a gene target through which ezetimibe acts is important to understanding the process of cholesterol absorption and to the development of other, novel absorption inhibitors. The present invention addresses this need by providing a rat and a mouse homologue of human NPC1L1 (also known as NPC3; Genbank Accession No. AF192522; Davies, et al., (2000) Genomics 65(2): 137-45 and Ioannou, (2000) Mol. Genet. Metab. 71(1-2): 175-81), an ezetimibe target.
NPC1L1 is an N-glycosylated protein comprising a YQRL (SEQ ID NO: 38) motif (i.e., a trans-golgi network to plasma membrane transport signal; see Bos, et al., (1993) EMBO J. 12: 2219-2228; Humphrey, et al., (1993) J. Cell. Biol. 120: 1123-1135; Ponnambalam, et al., (1994) J. Cell. Biol. 125: 253-268 and Rothman, et al., (1996) Science 272: 227-234) which exhibits limited tissue distribution and gastrointestinal abundance. Also, the human NPC1L1 promoter includes a Sterol Regulated Element Binding Protein 1 (SREBP1) binding consensus sequence (Athanikar, et al., (1998) Proc. Natl. Acad. Sci. USA 95: 4935-4940; Ericsson, et al., (1996) Proc. Natl. Acad. Sci. USA 93: 945-950; Metherall, et al., (1989) J. Biol. Chem. 264: 15634-15641; Smith, et al., (1990) J. Biol. Chem. 265: 2306-2310; Bennett, et al., (1999) J. Biol. Chem. 274: 13025-13032 and Brown, et al., (1997) Cell 89: 331-340). NPC1L1 has 42% amino acid sequence homology to human NPC1 (Genbank Accession No. AF002020), a receptor responsible for Niemann-Pick C1 disease (Carstea, et al., (1997) Science 277: 228-231). Niemann-Pick C1 disease is a rare genetic disorder in humans which results in accumulation of low density lipoprotein (LDL)-derived unesterified cholesterol in lysosomes (Pentchev, et al., (1994) Biochim. Biophys. Acta. 1225: 235-243 and Vanier, et al., (1991) Biochim. Biophys. Acta. 1096: 328-337). In addition, cholesterol accumulates in the trans-golgi network of npc1− cells, and relocation of cholesterol, to and from the plasma membrane, is delayed. NPC1 and NPC1L1 each possess 13 transmembrane spanning segments as well as a sterol-sensing domain (SSD). Several other proteins, including HMG-CoA Reductase (HMG-R), Patched (PTC) and Sterol Regulatory Element Binding Protein Cleavage-Activation Protein (SCAP), include an SSD which is involved in sensing cholesterol levels possibly by a mechanism which involves direct cholesterol binding (Gil, et al., (1985) Cell 41: 249-258; Kumagai, et al., (1995) J. Biol. Chem. 270: 19107-19113; Hua, et al., (1996) Cell 87: 415-426; and Radhakrishnan, A., et al., “Direct binding of cholesterol to the purified membrane region of SCAP: Mechanism for a sterol-sensing domain,” Mol. Cell. 15, 259-268 (2004)).